Non-aqueous electrolyte secondary cell using carbonaceous material for negative electrode

ABSTRACT

A material for a negative electrode of a cell, which is prepared according to a process of the present invention, can provides a cell having a high true specific gravity, a high charging capacity and an excellent cycle characteristic. The process of the present invention comprises the steps of carbonizing an organic compound to form a carbide thereof, pulverizing said carbide to form a powder having an average particle size of 10 μm to 2 mm, and sintering said powder of the carbide at a temperature of 2,000° C. or higher to produce a graphite. In addition, in accordance with the present invention, there is also provided non-aqueous electrolyte secondary cell comprising a negative electrode, which is prepared by carbonizing an organic compound to form a carbide thereof, pulverizing the carbide to form a powder having an average particle size of 10 μm to 2 mm, and sintering the powder of the carbide at a temperature of 2,000° C. to form a graphite.

This is a continuation of application Ser. No. 08/647,755, filed May 15,1996, now U.S. Pat. No. 5,667,914.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a carbonaceous material suitable for theproduction of a negative electrode of a cell and a non-aqueouselectrolyte secondary cell using the carbonaceous material, and moreparticularly to a carbonaceous material suitable for the production of anegative electrode of a lithium ion-based secondary cell, and thelithium ion-based secondary cell provided with the negative electrodemade of such a carbonaceous material and having a high dischargecapacity and a high cyclic property.

2. Prior Art

In association with a recent remarkable progress of electronictechniques, reduction in size and weight of electronic devices has beenrealized continuously. There is therefore an increasing demand forportable power sources for such electronic devices, such as cells orbatteries, which has a smaller size, a lower weight and a higher energydensity than the conventional ones.

Hitherto, aqueous-electrolyte type cells such as lead batteries,nickel-cadmium cells and the like are predominately utilized asgeneral-purpose secondary cells. However, these secondary cells of suchan aqueous-electrolyte type are relatively satisfactory in cycleproperties thereof but insufficient in weight reduction and energydensity. In addition, they have posed an environmental problem.Accordingly, it is desirable to develop a novel cell system.

Under these circumstances, various studies and investigations have beenmade to develop an effective non-aqueous-electrolyte secondary cell(lithium ion-based secondary cells) in which lithium or an lithium alloyis used as a material for a negative electrode of the cell. Such alithium ion-based secondary cell has excellent advantages such as a highenergy density, a small self-discharge and a light weight.

However, the non-aqueous secondary cell has the following defect. Thatis, metal lithium is eluted from or deposited on the negative electrodein a charge/discharge cycle of the cell, so that a dendrite-like crystalwhich has grown on the negative electrode, reaches a positive electrode.That is, there is a possibility that a short circuit is caused withinthe cell. A probability of occurrence of the short circuit is increased,particularly as a charge/discharge cycle of the cell proceeds. Such ashort circuit causes problems concerning safety and reliability, wherebya practical use of the cell is considerably prohibited.

To overcome the afore-mentioned problems posed when the metal lithium isused as a material for the negative electrode of the cell, since 1991,there has been proposed and practically utilized a non-aqueouselectrolyte secondary cell (lithium ion-based secondary cell) whichemploys a negative electrode made of a carbonaceous material. Thenon-aqueous electrolyte secondary cell is worked according to aprinciple of a negative electrode reaction in which lithium is doped ina cavity between adjacent carbon atoms of the carbonaceous material ordedoped therefrom. When the cell is adequately designed, nocrystallization of metal lithium occurs even when the charge/dischargecycle proceeds. Thus, the properly designed cell exhibits a goodcharge/discharge cycle property and a high safety. In addition, the cellis excellent in rapid charging and discharging property andlow-temperature resistance.

Meanwhile, various carbonaceous materials usable for the negativeelectrode of the lithium ion-based secondary cell have been reported.Among them, a low-crystalline carbonaceous material which is produced bysintering an organic material such as cokes or glassy carbon at arelatively low temperature has been marketed and utilized at an earlierstage. As an electrolyte solution of the secondary cell whose negativeelectrode is made of the low-crystalline carbonaceous material, there isused a non-aqueous solvent composed primarily of propylene carbonatewhich is generally used in coin-shaped or cylindrical primary cells.

From a standpoint of imparting a high discharge capacity to the lithiumion-based secondary cell, there have been made various studies on amaterial for respective cell components such as a negative electrode anda positive electrode, as well as designing and charging/dischargingmethods. For example, one attempt has been made to use graphite as thematerial for negative electrode.

The graphite has a higher true specific gravity as compared with thelow-crystalline carbonaceous material, so that a raw mixture for thenegative electrode made of such a graphite shows a high packing density.Accordingly, the negative electrode material is advantageous inproviding the cell with a high energy density. However, the graphite hasnot been initially used as the material for negative electrode becauseit causes decomposition of the electrolyte solution made of propylenecarbonate in the charge/discharge cycle of the cell. Under thiscircumstance, it is conventionally considered that it would be difficultto use the graphite as the material for negative electrode. However, inrecent years, it has been found that, when ethylene carbonate is used asa main component of the electrolyte solution in place of propylenecarbonate, doping of lithium can be effectively performed withoutdecomposition of the electrolyte solvent. This is true even when thegraphite negative electrode having a highly-crystalline structure isused. Since 1994, a cell system employing a combination of the graphiteand ethylene carbonate have been commercialized.

However, when the graphite material is used as a material for thenegative electrode of the lithium ion-based secondary cell, there occursa problem that it exhibits a low cycle property as compared with thosein which a low-crystalline graphite material prepared by sintering thecarbonaceous material at a relatively low temperature of 2000° C. orlower is used for the negative electrode.

In general, charging of the lithium ion-based secondary cell is carriedout according to a constant voltage/constant current charging method inwhich a given charging voltage (upper charge voltage) and a givenmaximum charging current are employed.

In a lithium ion-based secondary cell whose negative electrode containsno metal lithium, lithium ions moved between the negative and positiveelectrodes in a charge/discharge cycle of the cell are provided by thosededoped from an active material of the positive electrode. The activematerial used as the positive electrode is specificallylithium-containing oxides or the like. The amount of the lithium ionsdedoped from the active material of the positive electrode is determinedby the voltage applied thereto, and the amount of lithium ions isincreased as the voltage becomes higher. Accordingly, in order to obtaina cell having a high discharge capacity, it is advantageous that thecell has the upper charge voltage as high as possible.

Furthermore, in the lithium ion-based secondary cell in which the highlycrystalline graphite material is used as the material of the negativeelectrode, there is a tendency that a high charging current causesdeterioration in cycle property of the cell. Accordingly, if thenegative electrode composed of such a graphite material is commerciallyused in the lithium ion-based secondary cell, it is desirable to employan upper charge voltage as low as 4.1 V, while it is general that anupper charge voltage of a charger is set to 4.2 V in the case of thelithium ion-based secondary cell whose negative electrode is composed ofa low-crystalline carbonaceous material.

In the lithium ion-based secondary cell employing the negative electrodecomposed of the graphite material, there is also a demand for increasingan upper charge voltage thereof to 4.2 V in order to achieve a higherdischarge capacity of the cell. By raising the upper charge voltage tosuch a level, the 4.2 V charger used for the cell with the negativeelectrode composed of the low-crystalline carbonaceous material, whichis now prevailing with a high reliability, can be applied to the cellhaving the graphite negative electrode, with a good interchangability.To meet this requirement, it is necessary to develop a graphite materialnot only having a maximum charging voltage of 4.2 V but also exhibitinga good cycle property.

OBJECT AND SUMMARY OF THE INVENTION

The present invention has been accomplished in view of theafore-mentioned problems encountered in the prior art.

It is therefore an object of the present invention to provide a processfor preparing a carbonaceous material suitable for a negative electrodeof a cell with a high true specific density and a highelectrode-charging property, whereby there can be obtained a cellshowing not only a maximum charging voltage as high as 4.2 V but also agood cycle property.

It is another object of the present invention to provide a non-aqueouselectrolyte secondary cell provided with a negative electrode composedof such a carbonaceous material.

In order to achieve the afore-mentioned objects, in one aspect of thepresent invention, there is a process for preparing a material for anegative electrode of a cell, including the steps of carbonizing anorganic compound to form a carbide thereof, pulverizing the carbide toform a powder having an average particle size of 10 μm to 2 mm, andsintering the powder of the carbide at a temperature of 2,000° C. orhigher to produce a graphite.

In another aspect of the present invention, there is provided anon-aqueous electrolyte secondary cell including a negative electrode,the negative electrode being prepared by carbonizing an organic compoundto form a carbide thereof, pulverizing the carbide to form a powderhaving an average particle size of 10 μm to 2 mm, and sintering thepowder of the carbide at a temperature of 2,000° C. to form a graphite.

These and other objects, features and advantages of the presentinvention will become more apparently from the following detaileddescription when read in conjunction with the accompanying drawings andthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross-sectional view of a non-aqueous electrolytesecondary cell according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Examples of the graphite material suitable for the production of anegative electrode of a non-aqueous electrolyte secondary cell generallyincludes a natural graphite and a synthetic graphite. The latter isprepared by carbonizing and graphitizing an organic compound.

In the present invention, there is used a synthetic graphite materialobtained by controlling production conditions thereof. The syntheticgraphite material is suitable for the production of a negative electrodewhich can exhibit a sufficient resistance when applied to acharge/discharge cycle at an upper charging voltage of 4.2 V or higher.

In the production of such a synthetic graphite material, various organiccompounds can be generally used as a starting material. The organiccompound is first carbonized at a temperature of 300° C. to 700° C. in agaseous stream containing an inert gas such as a nitrogen gas(carbonization process). A resultant carbide is then heated to atemperature of 900° C. to 1,500° C. at a rate of 1° C. to 100° C. perminute and allowed to stand at that temperature for 0 to 30 hours(calcination process). The calcined product is further heat treated at atemperature of 2,000° C. or higher, preferably 2,500° C. or higher(graphitization process) to obtain the aimed graphite material. Thethus-prepared graphite material is usually used in the from of powderfor the production of the negative electrode of the cell.

The formation of the powder (pulverization process) may be carried outafter any of a series of the afore-mentioned carbonization-,calcination- and graphitization-processes. However, from a standpoint ofhandling-easiness upon manufacture and enhancement of the graphitizationdegree, the pulverization process is usually carried out after thegraphitization process.

However, as a result of various studies and investigations made by thepresent inventors, it has been found that a characteristic of theresultant graphite material varies largely depending upon a grain sizeof the material used in the graphitization process even if the samestarting material and the same conditions concerning temperature, timeand atmosphere are employed.

In view of the above findings, in the present invention, the material tobe subjected to the graphitization process is pulverized in advance toform a powder having an average particle size of 10 μm to 2 mm.Incidentally, the average particle size means a 50% cumulative grainsize.

In case that a massive or shaped graphite material is subjected to thegraphitization process as done in many cases, the cell with the negativeelectrode composed of such a graphite material shows deterioration incycle property thereof, particularly when an upper charging voltage of4.2 V or higher is employed.

Even though the pulverization of the graphite material is carried out inadvance of the graphitization process, when the pulverized graphitematerial has an average particle size greater than 2 mm, there alsooccurs the problem that the cycle property of the cell with the negativeelectrode composed of such a graphite material is deteriorated at theupper charging voltage of 4.2 V or higher. Conversely, if the graphitematerial is pulverized to a fine powder having an average particle sizesmaller than 10 μm in advance of the graphitization process, theresultant graphitized powder correspondingly has a particle size of 10μm or smaller. If such a fine graphite powder is used for the negativeelectrode of the cell, a discharge capacity thereof is likely to bedeteriorated during storage, whereby a good shelf stability of the cellcannot be obtained.

On the other hand, when the graphite material is pulverized to a powderhaving an average particle size ranging from 10 μm to 2 mm and such afinely pulverized graphite material is subjected to the graphitizationprocess, the cell with the negative electrode composed of such agraphite material exhibits a good cycle property and an excellent shelfstability even when the cell is exposed to an upper charging voltage of4.2 V or higher. The average particle size of the material to besubjected to the graphitization process is preferably in the range of 15μm to 200 μm, more preferably 15 μm to 40 μm.

When the thus-produced graphite powder is actually applied to theproduction of the negative electrode, the average particle size of thegraphite be finally adjusted to preferably the range of 10 μm to 50 μm,more preferably 15 μm to 40 μm. If the average particle size of thegraphite powder used for the production of the negative electrodeexceeds 50 μm, there is a tendency that a heavy-load property of thecell is deteriorated. On the other hand, if the average particle size isless than 10 μm, a shelf stability of the cell is insufficient.Accordingly, when the average particle size of the graphite powder isgreater than the upper value of the afore-mentioned range, it isrequired that an additional pulverization process is conducted after thegraphitization process. However, if the graphite material having anaverage particle size ranging from 15 μm to 40 μm is already prepared inthe initial pulverization process which has been conducted before thegraphitization process, no further pulverization process is requiredbecause an average particle size of the finally produced graphite powdercan be also fallen within the range. The graphite powder meeting therequirement of the afore-mentioned grain size is considerably effectiveto produce a cell having an extremely high cycle property.

Meanwhile, in the pulverization process(es) to be conducted beforeand/or after the graphitization process, a classification treatment maybe simultaneously performed to obtain the graphite material having auniform grain size. The classification treatments may be performed byusing a conventional method such as a screen method, a pneumaticclassifying method or the like.

As described above, in the method according to the present invention,the pulverization process is incorporated between the carbonization andgraphitization processes in order to obtain the graphite powder capableof resisting the charge/discharge cycle in which an upper chargingvoltage of 4.2 V or higher is employed. In the pulverization process,the carbide formed in the carbonization process is pulverized to obtainthe graphite powder having an average particle size of 10 μm to 2 mm.

Incidentally, typical organic compounds usable as a starting materialfor the production of the graphite powder may include coals or pitches.

Specific examples of the pitches may include pyrolytic tars such as coaltar, ethylene bottom oil or crude oil, distillates prepared by usingvacuum distillation, atmospheric distillation or steam distillation,e.g., asphalt, thermal polycondensates, extracts, chemicalpolycondensates, dry distillates prepared from wood, or the like. Inaddition, the pitches may be those prepared from polymer compounds as astarting material, such as a polyvinyl chloride resin, polyvinylacetate, polyvinyl butyrate, 3, 5-dimethyl phenol resin, or the like.

In the course of the carbonization process, coals, the pitches and theorganic compounds is present in a liquid state up to about 400° C. Whenmaintained at the temperature range, these materials are subjected to acondensation between aromatic rings whereby a product composed of apolycyclic compound and having a laminated structure is obtained.Thereafter, when the product is heated to a temperature higher thanabout 500° C., a solid carbon precursor (semi-coke) is formed. Thisprocess is generally called a liquid phase carbonization process whichis typical to produce a graphitizable carbon.

Furthermore, compounds which are usable as the starting material for thegraphite material, may include polycyclic hydrocarbons such asnaphthalene, phenanthrene, anthracene, triphenylene, pyrene, perylene,pentaphene, pentacene or the derivatives thereof (which includecarboxylates, carboxylic acid anhydrides, carboxylimide and a mixturethereof), condensed heterocyclic compounds such as acenaphthylene,indole, iso-indole, quinoline, iso-quinoline, quinoxaline, phthalazine,carbazole, acridine, phenazine, phenatolidine or derivatives thereof, orthe like.

The afore-mentioned organic compounds are subjected to carbonization,pulverization and graphitization processes to produce graphite. Thepulverization process can be conducted under the afore-mentionedconditions. In addition, the carbonization and graphitization processescan be conducted under an environmental condition known in conventionalthermal treatment methods.

However, in the graphitization process, sintering of the raw material ispreferably carried out in an inert gas-containing atmosphere, morepreferably in an inert gas-containing stream which is passed through areaction system at a flow rate of 0.1 cm³ per minute or more based onone gram of the raw material. When the sintering of the raw material isconducted in such an inert gas-containing stream, volatile components iseffectively removed from the raw material so that the resultant graphiteexhibits an excellent lithium-doping ability. Furthermore, when thesintering is carried out under a vacuum-evacuating condition, removal ofthe volatile components in the raw material is promoted, whereby thegraphite produced can show more excellent lithium-doping ability.

The non-aqueous electrolyte secondary cell according to the presentinvention is manufactured by using the thus-produced graphite as amaterial for the negative electrode thereof.

The graphite powder used for the production of the negative electrode ofthe cell preferably has a true specific gravity of 2.10 g/cm³ or more,more preferably 2.18 g/cm³ or more to achieve a high packing density ofthe electrode.

In order to obtain the graphite powder having such a high true specificgravity, it is desired that the graphite satisfies the followingrequirements concerning an interplanar spacing of 002 plane, a thicknessof crystallite along c-axis, a bulk specific gravity, an average shapeparameter X_(ave), and a G value according to Laser-Raman spectroscopy.

That is, the interplanar spacing of 0002 plane is in the range of 0.335nm to 0.34 nm (inclusive of both values), preferably 0.335 nm to 0.337nm (inclusive of both values). The thickness of crystallite along c-axisis preferably not less than 16.0 nm, more preferably not less than 24.0nm.

The bulk specific gravity, which is a value obtained according to JISK-1469, is 0.3 g/cm³ or more.

The average shape parameter X_(ave) is desirably not more than 125.Incidentally, a shape parameter (x) is calculated by the followingequation:

    x=(L/T)×(W/T)

where x represents the shape parameter; T is a thickness of a thinnestpart of the powder; L is a length of a major axis of the powder; and Wis a length of the powder along the direction perpendicular to the majoraxis.

The average shape parameter X_(ave) means an average value of the shapeparameters (x) and is obtained by using the following method.

First, a specimen of a graphite powder is observed using a scanningelectron microscope (SEM) to select 10 particles whose particle sizesare within the range of ±30% of the average particle size of the entiregraphite powder when measured by means of a grain-size distributionmeasuring apparatus used in a laser-diffraction method or the like. Theshape parameters (x) of the selected ten particles are calculated byusing the afore-mentioned equation to obtain an average value thereof.

The Laser-Raman spectroscopy is a measuring method which reflectsinformation concerning oscillation of a crystal structure of thecarbonaceous material at a high sensitivity. The G value obtained by theRaman spectroscopy is an index useful to evaluate a micro-structuraldefect and represents a ratio of an integrated intensity of Raman banddue to an amorphous structure to that due to a complete graphitestructure in the carbonaceous material. The G value is preferably 2.5 ormore. If the G value is less than 2.5, it is not necessarily assuredthat the graphite has a true specific gravity of 2.1 g/cm³ or more.

In addition to the afore-mentioned parameters of the crystal structureand the shape parameter, the resultant graphite preferably has adischarge capacity of 250 mAh/g or higher, preferably 270 mAh/g orhigher when measured at the first cycle by an intermittent charging anddischarging method, as described hereinafter.

On the other hand, the preferred materials for the positive electrode ofthe cell are those containing a large amount of dedopable and dopablelithium. One example of such a material for the positive electrode islithium/transition metal composite oxides which is represented by thegeneral formula of Li_(x) Ni_(y) Co_(1-y) O₂ wherein 0.05≦×≦1.10, andtherefore contains at least one of nickel and cobalt.

Such a lithium/transition composite oxide can be prepared by mixinglithium and a hydroxide, an oxide or a carbonate of transition metalsuch as cobalt or nickel with each other at adequate proportions andthen sintering the mixture at a temperature of 60 to 1,000° C.

The electrolyte solution used in the cell according to the presentinvention, may be a solution prepared by dissolving a lithium salt in anon-aqueous solvent.

In this case, since the negative electrode of the cell according to thepresent invention is composed of the graphite material, the use ofpropylene carbonate, which is likely to be decomposed by graphite,should be avoided. Instead, the electrolyte solution preferablycontains, as one of main components of the non-aqueous solvent, ethylenecarbonate. In view of various characteristics of the cell, a combinationof plural solvents as described below is desirably used.

For instance, chain-like esters are desirably used as a component of thesolvent, which can be combined with the ethylene carbonate, due to itshigh voltage resistance. Suitable chain-like esters may be carbonates,carboxylates, phosphates or the like. Especially, chain-like carbonatesare preferred. If these chain-like esters is mixed with the electrolytesolution, decomposition of the solvent is effectively prohibited in acharge cycle of the cell. The use of the chain-like esters also providesan enhanced electrical conductivity so that improved electrical currentcharacteristic of the cell can be obtained. Furthermore, asolidification point of the electrolyte solution is lowered by the useof the chain-like esters, which leads to an improvement inlow-temperature characteristic of the cell and lowering of thereactivity with metal lithium whereby a high safety of the cell can beachieved.

Specific examples of the afore-mentioned chain-like carbonates mayinclude asymmetric chain-like carbonates such as methylethyl carbonate(MEC), methylpropyl carbonate (MPC), and mixture solvents containing theasymmetric chain-like carbonate, such as a mixture solvent ofmethylethyl carbonate and dimethyl carbonate (DMC) or a mixture solventof methylethyl carbonate and diethyl carbonate (DEC). In addition, amixture solvent containing symmetric chain-like carbonate, such as acombination of dimethyl carbonate and diethyl carbonate, can berelatively suitably used for this purpose.

The mixing ratio of ethylene carbonate to components other than theethylene carbonate in the solvent is preferably in the range of 7:3 to3:7 on a volume basis.

The components other than the ethylene carbonate may be composed of aplurality of compounds. In case the components are composed of a mixtureof methylethyl carbonate (MEC) and dimethyl or diethyl carbonate (DMC orDEC), the mixing ratio of methylethyl carbonate (MEC) to dimethyl ordiethyl carbonate (DMC or DEC) is preferably in the range of 2:8 to 9:1.In case the components is composed of a mixture of dimethyl carbonate(DMC) and diethyl carbonate (DEC), the mixing ratio of dimethylcarbonate (DMC) to diethyl carbonate (DEC) is preferably in the range of1:9 to 9:1.

An electrolyte dissolved in the electrolyte solution is any electrolyteused in this type of cell. Examples of the electrolytes may includeLiClO₄, LiAsF₆, LiPF₆, LiBF₄, LiB(C₆ H₅)₄, LiCl, LiBr, LiSO₃ CH₃, LiSO₃CF₃, LiN(SO₂ CF₃)₂, LiC(SO₂ CF₃)₃, or the like. The preferredelectrolyte is LiPF₆.

The graphite material used for the negative electrode of the cell isproduced by carbonizing the organic compound and then sintering theresultant carbide at an elevated temperature of 2,000° C. or higher, asdescribed above. In the method according to the present invention, thecarbide is preliminarily pulverized to form a powder of the carbidehaving an average particle size of 10 μm to 2 mm before subjected to thegraphitization process.

When the negative electrode is formed from the graphite powder thusprepared through the afore-mentioned processes including thepulverization process, the cell having the negative electrode with ahigh packing density can be obtained. In addition, even when an uppercharging voltage of the cell is adjusted to 4.1 V or more, especially4.2 V or more, the cell can exhibit a good cycle property. Thus, thecell does not show any inconvenience at a high upper charging voltage.Accordingly, the cell can be improved in energy density.

Furthermore, if the graphite powder has a final average particle size of10 μm to 50 μm, the cell has a good shelf stability and a goodheavy-load property.

EXAMPLES

The present invention is described in detail below by way ofexperimental examples.

Preparation of Material for Negative Electrode:

A material for a negative electrode was prepared in the followingmanner.

A petroleum pitch was calcinated at a temperature of 1,200° C., and thenpulverized. The pulverized material was sintered (graphitized) at atemperature of 3,000° C. in an inert gas-containing atmosphere. Thegraphitized product was pulverized again to form a synthetic graphitepowder. Thus, graphite powder specimens Nos. 1 to 9 were obtained. Amongthe specimens, the graphite powder specimens Nos. 1 and 2 were preparedwithout the earlier pulverization process before the graphitizationprocess while the graphite powder specimens Nos. 6 to 9 were preparedwithout the later pulverization process after the graphitizationprocess.

Average particle sizes of the specimen Nos. 1 to 9 before and after thegraphitization process are shown in Table 1.

Incidentally, in the event that the graphite powder had an averageparticle size less than about 100 μm, the average particle size of eachgraphite powder specimen was determined by using a laserdiffraction-type grain distribution-measuring apparatus. The averageparticle size is a value on a volume basis and represents a 50%cumulative particle size of the powder. On the other hand, in the eventthat the graphite powder had an average particle size of about 100 μm ormore, a scanning electron microscope (SEM) was used to determine theaverage particle size of each graphite powder specimen. In the lattercase, the average particle size was obtained only as an approximatevalue.

                  TABLE 1    ______________________________________                        Average          Final              Pulveri-  particle  Pulveri-                                         average    Graphite  zation    size      zation particle    powder    before    before    after  size after    specimen  graphiti- graphiti- graphiti-                                         graphiti-    No.       zation    zation    zation zation    ______________________________________    1         No        10 mm     Yes    31 μm    2         No         2 mm     Yes    30 μm    3         Yes       0.5 mm    Yes    63 μm    4         Yes       0.2 mm    Yes    48 μm    5         Yes       0.1 mm    Yes    38 μm    6         Yes       32 μm  No     29 μm    7         Yes       18 μm  No     15 μm    8         Yes       12 μm  No     11 μm    9         Yes        5 μm  No      5 μm    ______________________________________

The graphite powder specimen No. 6 selected as a typical example of thegraphite powder, was subjected to a powder X-ray diffraction measurementto obtain an interplanar spacing of (002) plane and a thickness ofcrystallite along c-axis thereof. In addition, a G value of the graphitepowder specimen was obtained by a Laser-Raman spectroscopy and a truespecific gravity thereof was obtained by using a pycnometer 23 method(n-butanol immersion method).

As a result, it was confirmed that the interplanar spacing of (002)plane was 0.337 nm and the thickness of crystallite along c-axis was 30nm. In addition, the G value according to the Laser-Raman spectroscopywas 13.6 and the true specific gravity according to the pycnometermethod was 2.22.

Next, a test electrode was prepared by using the graphite powderspecimen No. 6. The negative electrode was incorporated into a testcell, which was then measured to obtain its discharge capacity per onegram.

Meanwhile, the test electrode was prepared in the following manner.

Immediately before the preparation of the negative electrode, theabove-prepared graphite powder was preliminarily heated up to atemperature of 600° C. at a temperature rise rate of about 30° C. perminute in an argon atmosphere and allowed to stand for 1 hour at thattemperature. The graphite powder was then cooled to room temperature.The thus-prepared carbonaceous powder material was mixed with 10% byweight of polyvinylidene fluoride as a solvent. The resultant mixturewas dried to prepare a test mixture. 37 mg of the test mixture wasshaped, together with a nickel net serving as a current collector, intoa pellet having a diameter of 15.5 mm to prepare the test electrode.

The thus-prepared test electrode was accommodated in a cell casing. Thecell casing was then fitted in a mating manner through a separator to anelectrode cap in which a counter electrode (metal lithium) was mountedso that the test electrode (as a working electrode), the separator andthe counter electrode were arranged in an overlapped and laminatedrelation to each other. After the respective electrodes were impregnatedwith the electrolyte solution, peripheral mated edges of the cell casingand the electrode cap were caulked through a seal gasket to prepare acoin-shaped test cell having a hermetically sealed interior. Materialsfor the counter electrode, the separator and the electrolyte solutionand the dimension of the cell were as follows. Incidentally, theafore-mentioned cell production was carried out in the dry air having adew point of -40° C. or lower.

Constitution of Test Cell:

Cell dimension: Coin-shape having a diameter of 20 mm and a thickness of2.5 mm;

Counter electrode: Metal lithium;

Separator: Polypropylene porous membrane;

Electrolyte solution: Solution dissolving 1 mole/l of LiPF₆ in a mixturesolvent of ethylene carbonate and diethyl carbonate (volume ratio=1:1).

The thus-produced test cell was subjected to a charge/discharge cycle inwhich a discharge capacity thereof was measured by using an intermittentcharging and discharging method.

The charge/discharge cycle according to the intermittent charging anddischarging method is described in detail below. Strictly speaking, inthe intermittent charging and discharging method, a process in whichlithium is doped in the carbonaceous material is a discharge cycle and aprocess in which lithium is dedoped from the carbonaceous material is acharge cycle. However, hereinafter, for the sake of convenience in viewof commercially available cell products, the former process in whichlithium is doped in the carbonaceous material is called a "charge" cyclewhile the latter process in which lithium is dedoped from thecarbonaceous material is called a "discharge" cycle.

First, the test cell was charged at a constant current of 0.5 mA for onehour, followed by two hour-interruption. The one hour-charge/twohour-interruption cycle was repeated until an equilibrium potentialestimated by plotting a change in cell voltage every -1/2 hour in theinterruption period, reached 3 to 15 mV (Li/Li+). Successively, the cellwas discharged at a constant current of 0.5 mA for one hour, followed bytwo hour-interruption. The one hour-discharge/two hour-interruptioncycle was repeated until a terminal voltage of the cell reached 1.5 V toobtain a total discharge capacity of the cell. Based on thethus-obtained total discharge capacity, a discharge capacity per onegram of the carbonaceous material was calculated. As a result, it wasconfirmed that the cell employing the carbonaceous material had adischarge capacity of 300 mAh/g.

Preparation of Material for Positive Electrode:

A material for the positive electrode was prepared in the followingmanner.

Lithium hydroxide and cobalt oxide were mixed with each other such thatthe atomic ratio of lithium to cobalt was 1:1. The mixture was thensintered in an oxygen-containing atmosphere at a temperature rangingfrom 700° C. to 800° C. for 12 hours. The resultant sintered product waspulverized and then subjected to a X-ray diffraction measurement. Theresult of the measurement revealed that a peak of plots in the X-raydiffraction of the sintered product was coincident with a peak of LiCoO₂recorded in JCPD file. Thus, it was confirmed that the sintered productwas LiCoO₂.

Preparation of Test Cell:

Next, by using the above-prepared materials for the negative andpositive electrodes, a lithium ion-based secondary cell was produced.The thus-produced cell is shown in FIG. 1.

The negative electrode 1 of the cell was produced in the followingmanner.

Each of the afore-mentioned graphite powder specimens Nos. 1 to 9 wasused as the material for the negative electrode of the cell. 90 parts byweight of the graphite powder was mixed with 10 parts by weight of avinylidene fluoride resin as a binder to prepare a mixture for thenegative electrode. The mixture was dispersed in N-methyl-2-pyrrolideneas a solvent to form a pasty slurry. The pasty slurry was coated onopposite surfaces of a band-like copper foil serving as a negativeelectrode current collector 10 and having a thickness of 10 μm, driedand pressure-formed into the band-like negative electrode 1.

The positive electrode 2 was prepared in the following manner.

91 parts by weight of LiCoO₂ prepared above, 6 parts by weight ofgraphite powder serving as a conductive material and 3 parts by weightof a vinylidene fluoride resin as a binder were mixed with each other toprepare a mixture for the positive electrode. The mixture was thendispersed in N-methyl-2-pyrrolidene as a solvent to form a pasty slurry.The pasty slurry was coated on opposite surfaces of a band-like aluminumfoil serving as a positive electrode current collector 11 and having athickness of 20 μm, dried and pressure-formed into the band-likepositive electrode 2.

The band-like negative electrode 1, the band-like positive electrode 2and the separators 3 made of a finely-porous polyolefin film werelaminated such that the negative electrode 1, the first separator 3, thepositive electrode 2 and the second separator 3 were overlapped in thisorder. The thus-prepared laminate was rolled to form many layers fromcenter to the outside and an outer free end of the laminate was fixedonto an outer surface of the rolled laminate by means of an adhesivetape so that a roll electrode having an outer diameter of 18 mm wasobtained.

The roll electrode was accommodated in a nickel-plated iron cell casing5. Insulating plates 4 were attached to opposite end faces of the rollelectrode in the cell casing. An aluminum lead wire 13 was connected atone end thereof to the positive electrode current collector 11 andwelded at the other end thereof onto a projection of a safety valve 8electrically connected to the cell lid 7, while a nickel lead wire 12was connected at one end thereof to the negative electrode currentcollector 10 and welded at the other end thereof onto a bottom of thecell casing 5.

Poured into the cell casing 5 accommodating the roll electrode was theelectrolyte solution prepared by dissolving 1 mole/l of LiPF₆ in amixture solvent composed of ethylene carbonate and methylethylcarbonate. Peripheral edges of the cell casing 5 were caulked togetherwith insulating seal gaskets placed thereon whereby the safety valve 8having a current-shut-off mechanism, a positive temperature coefficient(PTC) element 9 and the cell lid 7 were fixed together in the cellcasing 5 and a hermetically sealed interior is formed in the cell. Inthe afore-mentioned cell production process, the test cells Nos. 1 to 9,which were cylindrical lithium ion-based secondary cells and each had adiameter of 18 mm and a height of 65 mm, were prepared.

Discharge capacity, charge/discharge cycle property, heavy-load propertyand shelf stability of the thus-prepared test cells were evaluated asfollows.

Discharge Capacity:

Each test cell was charged for 3 hours by setting an upper chargevoltage to 4.2 V and a charge current in constant current region to 1 A.Thereafter, the cell was discharged at a constant current of 0.2 A untilthe cell voltage was decreased to 2.75 V to measure a discharge capacitythereof.

Charge/Discharge Cycle Property:

Each test cell was charged for 3 hours by setting an upper chargevoltage to 4.2 V and a charge current in constant current region to 1 A.Thereafter, the test cell was discharged at a constant output of 2.5 Wuntil the cell voltage was decreased to 2.5 V. The charge/dischargecycle was repeated to measure the discharge capacities at the 1st and200th cycles, which were each obtained at the time when the cell voltagewas decreased to 2.75 V. The charge/discharge cycle property wasdetermined as a ratio of the discharge capacity at the 200th cycle(discharge capacity 2) to that at the 1st cycle (discharge capacity 1),which was specifically given by the following equation:

     discharge 2!/ discharge 1!×100(%)

Heavy-Load Property:

Each test cell was charged for 3 hours by setting an upper chargevoltage to 4.2 V and a charging current in constant current region to 1A. Thereafter, the test cell was discharged at a constant current of 0.2A until the cell voltage was decreased to 2.75 V to measure a dischargecapacity of the cell (discharge capacity 3). After charged under thesame conditions as described above, the test cell was discharged at aconstant current of 3 A until the cell voltage was decreased to 2.75 Vto measure a discharge capacity of the cell (discharge capacity 4). Theheavy-load property was determined as a ratio of the discharge capacity4 to the discharge capacity 3, which was specifically given by thefollowing equation:

     discharge 4!/ discharge 3!×100(%)

Shelf Stability:

Each test cell was charged for 3 hours by setting an upper chargevoltage to 4.2 V and a charge current in constant current region to 1 A.Thereafter, the test cell was discharged at a constant output of 2.5 Wuntil the cell voltage was decreased to 2.5 V, during which a dischargecapacity at the time when the cell voltage was decreased to 2.75 V wasmeasured. The discharge capacity was referred to as "discharge capacitybefore storage." Separately, the test cell was charged under the sameconditions as described above and stored for one month at an ambienttemperature of 45° C. After the one-month storage, the test cell wasdischarged at a constant output of 2.5 W until the cell voltage wasdecreased to 2.5 V. Successively, the test cell was subjected to fivecharge/discharge cycles under the same conditions as described above. Ateach cycle, the discharge capacity (referred to as "discharge capacityafter storage") was measured to obtain a ratio thereof to the dischargecapacity before storage, which was specifically given by the followingequation:

     discharge capacity after storage!/ discharge capacity before storage!×100(%)

The maximum value of the thus-obtained ratios was determined to be adischarge capacity restoration rate of the cell.

The results of the afore-mentioned measurements are shown in Table 2 aswell as the kind of the graphite powder used as a material for thenegative electrode of each cell.

                  TABLE 2    ______________________________________                                     Heavy-         Graphite   Discharge                             Cycle   load  Shelf    Cell specimen   capacity property                                     property                                           stability    No.  No.        (Ah)     (%)     (%)   (%)    ______________________________________    1    1          0.142    65      81    92    2    2          0.143    80      82    90    3    3          0.134    78      66    94    4    4          0.137    82      73    93    5    5          0.141    83      78    93    6    6          0.143    85      81    92    7    7          0.147    86      83    88    8    8          0.148    87      85    86    9    9          0.149    89      88    83    ______________________________________

It was noted from Table 2 that the test cells Nos. 2 to 9 in which thegraphite powder used had an average particle size of 2 mm or less beforethe graphitization process, exhibited a good cycle property ranging fromabout 80% to about 90%. On the other hand, the test cell No. 1 in whichthe graphite powder used had an average particle size of 10 mm,exhibited an insufficient cycle property as low as 65%.

This indicated that the grain size of the material in the graphiteproduction process, especially in the graphitization process, gave alarge influence on the cycle property of the resultant cell. As aresult, it was confirmed that, when the graphite powder before thegraphitization process had an average particle size of 2 mm or less, thecycle property of the cell was improved.

Among the test cells Nos. 2 to 9, the test cell No. 9 contained thecarbonaceous material having an average particle size as small as 5 μmbefore the graphitization process, which results in producing a finalgraphite powder having the same average particle size as fine as 5 μm.The test cell was superior in cycle property but inferior in shelfstability as compared with those of other test cells. In addition, evenin case the graphite material was subjected to the pulverization processbut had a final average particle size as large as 63 μm, for example,that used for the test cell No. 3, it was confirmed that the test cellusing such a graphite powder was inferior in heavy-load property ascompared with other test cells.

In consequence, in order to satisfy all the requirements concerningcycle property, heavy-load property and shelf stability, the averageparticle size of the carbonaceous material before the graphitizationprocess should be in the range of 10 μm to 2 mm and the average particlesize of the finally produced graphite powder should be in the range of10 μm to 50 μm.

Furthermore, detailed studies on the afore-mentioned results revealedthat the test cells Nos. 4 to 8 in which the carbonaceous powder havingan average particle size of 200 μm or less before subjected to thegraphitization process was employed, exhibited a good cycle property.Among them, the test cells Nos. 6 to 8, in which the carbonaceousmaterial having an average particle size of 40 μm or less before thegraphitization process was employed and therefore the resultant graphitepowder was not pulverized after the graphitization process, exhibited aconsiderably excellent cycle property. The test cells Nos. 2 to 7, inwhich the graphite material having an average particle size of 15 μm ormore before and after the graphitization was used, showed a good shelfstability. The test cells Nos. 5 to 8, in which the graphite materialhaving a final average particle size of 40 μm or less was used, showed agood heavy-load property.

As a result, it was confirmed that, in order to obtain cells having anexcellent characteristics, the average particle size of the carbonaceouspowder before the graphitization process is preferably in the range of15 μm to 200 μm, more preferably 15 μm to 40 μm, and the final averageparticle size of the graphite powder after the graphitization process ispreferably in the range of 15 μm to 40 μm.

What is claimed is:
 1. A non-aqueous electrolyte secondary cellcomprising:a positive electrode including a lithium/transition metalcomposite oxide material; a negative electrode including a graphitematerial prepared by carbonizing an organic compound to form a carbidethereof, pulverizing said carbide to form a powder having an averageparticle size of 10 μm to 2 mm, and sintering said powder of the carbideat a temperature of 2000° C. or higher to form said graphite; and anelectrolyte solution comprising a lithium salt dissolved in anon-aqueous solvent.